1. Field of the Invention
The present disclosure is generally related to communication systems, and, more particularly, is related to wireless communication systems and methods.
2. Related Art
Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), orthogonal frequency division multiplex (OFDM), or some other multiplexing techniques. OFDM systems may provide high performance for some channel environments. FIG. 1A is a block diagram that illustrates an exemplary single-in, single-out (SISO) orthogonal frequency division multiplexing (OFDM) communication system 100 (herein, SISO system 100) that is compliant with IEEE 802.11 standards. The SISO system 100 comprises a transmitter device 102 and a receiver device 104. The transmitter device 102 comprises a transmit (TX) processor 106, radio circuitry 108, and antenna 110. The receiver device 104 comprises an antenna 112, radio circuitry 114, and receive (RX) processor 116.
The transmitter device 102 comprises well-known circuitry that divides the high-speed data signals into tens or hundreds of lower speed signals and transmits the signals in parallel over respective frequencies within a radio frequency (RF) signal that comprise subcarrier frequencies (“subcarriers”). The frequency spectra of the subcarriers overlap so that the spacing between them is minimized. The subcarriers are also orthogonal to each other so that they are statistically independent and do not create cross-talk or otherwise interfere with each other. FIG. 1B is a schematic diagram that illustrates an exemplary OFDM symbol 118 corresponding to signals processed in the SISO system 100. In 802.11 standards, each OFDM symbol 118 provided by the transmitter device 102 comprises 52 subcarriers (partially shown for brevity) centered at a defined reference or carrier frequency, with a bandwidth (BW) of approximately 20 mega-Hertz (MHz). The spectrum resulting from processing at the receiver device 104 is typically centered at the same reference or carrier frequency.
In operation, the transmit processor 106 receives data signals (designated as TX data1 at a defined data rate designated as TX Rate1). The transmit processor 106 encodes and interleaves the data and maps the interleaved data into respective subcarrier channels as frequency domain symbols. Further processing by the transmit processor 106 may result in the insertion of training signals, cyclic extensions (e.g., guard intervals), and additional processing such as inverse fast Fourier transformations (IFFT) and wave shaping. The processed subcarriers are provided to the radio circuitry 108, which provides filtering, modulation, amplification, and upconversion functionality, ultimately resulting in the transmission of data from antenna 110.
FIG. 1C is block diagram that describes an exemplary OFDM packet structure 150 used in the transmission of information between the transmitter device 102 and the SISO receiver device 104. Additional information about the packet structure can be found in 802.11 standards. The packet structure 150 is generated in a baseband processing section (e.g., in or in cooperation with an inverse fast Fourier transform (IFFT) operation) of the transmitter device 102, and comprises several sections. Sections A and B are comprised of short training symbols (STS). Section A is used by a communication system to provide signal detection, automatic gain control (AGC), and diversity selection functionality. Section B is used by a communication system to provide coarse frequency offset estimation and timing synchronization. Section C, sometimes referred to as a long training symbol (LTS), is used by a communication system to provide channel estimation and fine frequency offset estimation. Sections A-C are typically referred to as the preamble portion of a packet. Section D is referred to as the signal field or header, and contains data rate and packet length information. Sections E and F are OFDM symbols, such as OFDM symbol 118a. Sections D, E, and F provide rate length, service and data, and data, respectively.
At the receiver device 104, the antenna 112 receives the transmitted data, which is provided to radio circuitry 114 to complement the processing that occurred at radio circuitry 108. The data is then provided to receive (RX) processor 116, which provides clock recovery, cyclic extension removal, transformations (e.g., fast Fourier transformation, FFT), demapping, deinterleaving, and decoding to recover the TX data1 as RX data1. Transmitter and receiver devices that are compliant to IEEE 802.11a/g standards, such as shown in FIG. 1A, are often referred to as legacy radios or legacy devices.
Continual demand for increased data rates has resulted in the advancement of communications system technology, such as the use of multiple antennas in a single device having transmitter and/or receiver functionality. In terrestrial communication systems (e.g., a cellular system, a broadcast system, a multi-channel multi-point distribution system (MMDS), among others), a RF modulated signal from a transmitter device may reach a receiver device via a number of transmission paths. The characteristics of the transmission paths typically vary over time due to a number of factors such as fading and multi-path. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used for data transmission. Spatial multiplexing refers to a technique where a transmission channel is divided into multiple “spatial channels” through which independent streams can be transmitted and received via multiple transmit and receive antennas, respectively.
FIG. 2 is a block diagram that illustrates a multiple-input multiple-output (MIMO) OFDM communication system 200 (herein, MIMO system 200). The MIMO system 200 employs multiple transmit antennas and multiple receive antennas for data transmission. Through spatial multiplexing, a MIMO channel formed by the transmit and receive antennas may be decomposed into independent channels. Each of the independent channels is also referred to as a spatial subchannel of the MIMO channel. The MIMO system 200 comprises a transmitter device 202 and receiver device 204. The transmitter device 202 comprises transmit (TX) processors 206 and 212, radio circuitry 208 and 214, and antennas 210 and 216. The receiver device 204 comprises antennas 218 and 226, radio circuitry 220 and 228, and receive (RX) processors 224 and 230. The transmit processors 206 and 212 and the radio circuitry 208 and 214 comprise similar circuitry to that found in and described for transmit processor 106 (FIG. 1A), with the addition of circuitry for implementing spatial multiplexing. The radio circuitry 220 and 228 and receive processors 224 and 230 also share common circuitry with like components shown in and described for receiver device 104 (FIG. 1A). The receive processors 224 and 230 may comprise signal separating functionality to remove interference caused by multiple transmit signals occupying the same bandwidth at the receive antennas 218 and 226, and thus may be used to increase the data rate.
In developing systems such as MIMO that utilize multiple-antenna devices, there is a need to consider legacy receivers (e.g., single-input, single output (SISO), OFDM receivers) and the design challenges concomitant with implementing transmitters with multiple antennas in an environment that still uses legacy receivers.